1. Using the su command

Many commands can only be run as the root user so to run these commands we need to become "root". To do this, we can use the su command (substitute user). The su command takes the following format:

su - <user>

or

su <user>

but most commonly we will use su to become the root user:

su - root

or

su root

If no username is specified, then the root user is assumed, so the above is often shortened to:

su -

or

su

but the two commands above behave differently. 'su <user>' gives the current user the identity of <user> whereas 'su - <user>' gives the current user the identity of <user> together with <user>'s environment that would be obtained by logging in as <user>.

Often a user will become root using just 'su', try to run a command (eg, ifconfig), and get a 'command not found' error. For example:

su
Password:
ifconfig
bash: ifconfig: command not found

The reason is that regular system users and the root user have different PATH environment variables. When you type a Linux command, the shell will search the user's PATH to try to locate the command to run. It starts searching each directory on the PATH until a match is found.

Often when a person reports a problem, in IRC or otherwise, they are referred to this page. In debugging WHY a given binary cannot be seen, it is helpful to view the currently effective PATH with: echo $PATH

Commands for regular users are mostly located in /usr/bin, and /bin and occasionally /usr/local/bin -- the /usr/local/* path prefix is not used for packaging by default upstream. However, root commands are mostly located in /usr/sbin, and /sbin and occasionally /usr/local/sbin As such, root's PATH reflects this.

When you become root by using 'su -', you also adopt root's PATH whereas using just 'su' retains the original user's PATH, hence why becoming root using just 'su' and trying to run a command located in /usr/local/sbin, /usr/sbin, or /sbin results in a 'command not found' error. For a more detailed explanation, see the bash manual page (man bash), particularly the section on INVOCATION and login shells.

So you either need to specify the full PATH to the command if you just used 'su' (eg, /sbin/ifconfig) or use 'su -' when becoming root.

2. Using sudo

You don't need to be root every time you want to run some specific administrative tasks. Thanks to sudo, you can run some or every command as root. Once sudo is installed (package name: sudo), you can configure it by running 'visudo' as root. Basically, it runs $EDITOR (vim as default) on /etc/sudoers, but it is not recommended to do it manually. If you are on a desktop computer, you will want to be able to do almost everything.

So, the quick and dirty way to use sudo would be to add at the end of the sudoers file :

bob ALL=(ALL) ALL

where bob is the name of the user. Save (press escape, then type ZZ), and you are ready to go. Log in as bob, and run for example:

$sudo yum update

sudo will ask for a password. This password is bob's password, and not root's password, so be careful when you give rights to a user with sudo.

But sudo can do more. We can allow a user or group of users to run only one command, or a group of commands. Let's go back to our sudoers file (which is, by the way, well commented on CentOS 5). Let's start with bob and alice, members of a group named admin. If we want all users of the "admin" group to be able to run every command as root, we can modify our example:

%admin ALL=(ALL) ALL

bob can still do his stuff, and alice is now allowed to run sudo, with the same rights, with her password. If bob and alice are not in the same group, we can define a user alias in the sudoers file:

User_Alias ADMINS = alice, bob

here we define an alias named ADMINS, with alice and bob as members.

However, we don't want alice and bob to run every command as root, we want them to run only updatedb. Let's define a command alias:

Cmnd_Alias LOCATE = /usr/sbin/updatedb

But it's not enough! We need to tell sudo the users defined in ADMINS can run the commands defined in LOCATE. To do this, we replace the line with "%admin" with this line:

ADMINS ALL = LOCATE

it means that users of alias ADMINS can run ALL the commands in the LOCATE alias.

alice and bob should be able to run updatedb as root, by giving their password.

If we replace the last line of the file with:

ADMINS ALL = NOPASSWD: LOCATE

alice and bob can run "sudo updatedb" without entering a password.

It is possible to add more commands in a command alias and more aliases in the rule. For example, we can create an alias named NETWORKING containing some networking commands like ifconfig, route or iwconfig:

sudo is often used to give certain users partial access to privileged commands so that they can perform limited administrative tasks. One nice feature about sudo is that all sudo commands are logged in /var/log/secure. The above example will write this line in the log:

That's it. Now never forget, when using sudo: "with great power comes great responsibility".

2.1. Sudo shell

If you have sufficient rights configured in sudoers you can also open a root shell by using

sudo -s

or

sudo -i

The later works only on CentOS 5. if used as sudo -s you will get a shell running as root with the environment of the calling user. So be very careful as this will create or modify files as root but $HOME is set to the home directory of the calling user.

3. Graphical helpers

3.1. GNOME

To do...

3.2. KDE

The 'Run Command...' option on the KDE Menu has the facility (under Options) to run a command or application as a different user by supplying that users credentials. Users who are less confident at the command line may like to consider this as an alternative GUI-based method for tasks that require root privileges.

4. consolehelper

consolehelper is a wrapper for running GUI applications. When it starts, it checks configuration of PAM for the desired application. This means that we can authenticate user with all installed PAM modules. The basic method is to ask for a password, but if we have proper hardware, we can also authenticate using smartcards, tokens, fingerprints, etc. Configuration of PAM is beyond scope of this document (see PAM Administrator's Guide), so we will only show how to configure it when we want consolehelper to run an application as root and request the root password.

As an example, let's configure /usr/bin/xterm to launch as root.

First, because we also want to retain the ability to run xterm as a normal user, we create a symbolic link named /usr/bin/xterm-root to /usr/sbin/consolehelper:

ln -s /usr/sbin/consolehelper /usr/bin/xterm-root

Now to configure pam - create the file /etc/pam.d/xterm-root:

#%PAM-1.0
auth include config-util
account include config-util
session include config-util

Finally, configure consolehelper to run /usr/bin/xterm as the root user when started as 'xterm-root'. Create the file /etc/security/console.apps/xterm-root:

USER=root
PROGRAM=/usr/bin/xterm

That's it. Run 'xterm-root' (from the command line or from a .desktop file), enter the root password and enyoy. If you get an error: "Xlib: connection to ":0.0" refused by server", run 'xhost local:root' first or add the following line to the file /etc/security/console.apps/xterm-root